1. Field of the Invention
The present invention relates to communications systems that transmit signals that may be composed of one or more combined transmit carriers. Each of these carriers may include one or more communication channels. More particularly, the present invention relates to wireless communications systems and signal processing apparatus employed in wireless communications systems. The term ‘wireless communications systems’ includes cellular communication systems, personal communication systems (PCS), wireless local loop systems, and all other like systems.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and remote users are a key component of the modern communications infrastructure. These wireless systems are being placed under increasing performance demands that are taxing the capability of available equipment, especially wireless base station equipment. These increasing performance demands are due to both the increasing numbers of users within a given wireless region, as well as the bandwidth requirements allocated to wireless system service providers. The increasing number of wireless users is of course readily apparent and this trend is unlikely to slow due to the convenience of wireless services. The second consideration is largely due to the increased types of functionality provided by wireless systems, such as wireless Internet access and other forms of wireless data transfer over such systems. These considerations have resulted in a need for more communication channels per carrier and more carriers operating from each transmitting location of a wireless service network.
One method of transmitting multiple communication channels on a single carrier is to use a code multiplexed signal generator as shown in FIG. 1. Data channels from different users enter the code multiplexed signal generator 1 to produce a complex signal output represented by in-phase and quadrature-phase components V1 and V2 respectively. This complex signal output is then band limited by filtering 2, converted to a baseband analog signal by Digital-to-Analog (D/A) conversion 3, modulated to an RF frequency 4, amplified 5 and transmitted by an antenna 6. This method is used by wireless systems providing CDMA (Code Division Multiple Access) or WCDMA (Wideband Code Division Multiple Access) services.
Other methods exist for combining several communication channels onto a single carrier. For example the code multiplexed signal generator 1 in FIG. 1 could be replaced with a time multiplexed signal generator. As before multiple input data signals would be combined to produce a complex signal output represented by in-phase and quadrature-phase components V1 and V2 respectively. NADC (North American Digital Cellular) and GSM (Global System for Mobile Communications) wireless service providers use time multiplexed signal generators.
For single carrier generation, the signal generator 1 of FIG. 1 and the filter 2 create signal peaks which determine the peak-to-average ratio of the signal which must be D/A converted 3, modulated 4, and amplified 5. High peak-to-average ratios require increased cost in these components. D/A converters with large bit counts must be used to both create the large peaks and maintain adequate signal resolution to overcome noise generated in the D/A conversion process. High peak-to-average ratios require the use of very linear RF up converting modulator and power amplifier components to prevent signal distortion from increasing carrier bandwidth through distortion and intermodulation. Signal bandwidth is government regulated. Increased carrier bandwidth may cause operation outside government allocated operating bands in violation of the law.
FIG. 2 shows a prior art multiple carrier communication system. FIG. 2 shows signal generation of M complex signals Vm,l and Vm,2. Each complex signal would then be filtered 2, offset in frequency 7, and combined 8 to generate a single complex signal. This combined complex signal would then be processed in a manner identical to the single carrier signal after filtering 2 in FIG. 1.
When generating a multiple carrier signal as shown in FIG. 2, the output signal peak-to-average ratio is determined by the signal generators 1, the filters 2, and the interaction of each carrier in combining 8. This multiple carrier signal must then be D/A converted 3, modulated 4, and amplified 5. As with the single carrier, high peak-to-average ratios increase the cost of the D/A converter 3, RF up converting modulator 4, and amplifier 5 components.
In a previous approach, placing a signal-peak suppression block prior to filtering 2 has been employed in an attempt to reduce the peak-to-average ratio in single carrier communication systems. This prior art approach is shown in FIG. 3. The signal-peak suppression block 9 operates by adjusting the input complex signal prior to filtering. By making signal adjustments prior to filtering, the resultant adjustments do not affect signal bandwidth guaranteeing operation within government allocated limits. The subsequent filtering introduces new peaks, however, and the effectiveness of the prior signal-peak suppression block is greatly reduced.
As mentioned previously, the peak power of a multiple carrier communication system, as shown in FIG. 2, is dependent on the signal generators 1, filters 2, and interaction of each carrier in combining 8. The approach shown in FIG. 3 if inserted prior to filtering 2 in FIG. 2 would be incapable of correcting for the interaction of each carrier in combining. This limitation would eliminate most, if not all, benefits of such an application of prior art.
In single carrier communication systems it is often difficult to place a peak reduction block before filtering 2 and obtain effective peak reduction. Applying previous art in a multiple carrier communication system would prove ineffective. Therefore, a problem exists in prior approaches to reducing high signal peaks in communications systems.